U.S. Pat. No. 5,122,875-Raychaudhuri et al. describes a system for processing a high definition television (HDTV) signal subjected to MPEG-like variable length coding. MPEG is a standardized coding format being established by the International Organization for Standardization. The standard is described in the document "International Organization for Standardization," ISO/IEC DIS 11172, Coding for Moving Pictures and Associated Audio for Digital Storage Media, Rev. Nov. 23, 1991, which document is incorporated herein by reference for description of the general code format. The Raychaudhuri system includes a priority selector which separates codewords into high and low priority codeword sequences. The high and low priority codeword sequences correspond to compressed video data of relatively greater and lesser importance to image reproduction, respectively.
The priority selector analyzes codewords in order to calculate a breakpoint between high and low priority segments for a given image slice. As will be seen, a "slice" is an image segment. corresponding to one of the layers of the MPEG coding syntax. The codeword datastream is then passed to a transport processor which packs the codeword data into transport cells each including header and payload sections, and which provides output HP and SP datastreams.
In order to establish the priority breakpoint and consequently prioritized HP and SP bitstreams, the priority processing circuitry must have available slice codeword data statistics, unaffected by changes that may be caused by new codewords entering the priority processor during the next slice. One architecture for addressing this problem involves the use of two input priority analysis buffers, operating in "ping-pong" fashion whereby codewords for a current slice are being written into one buffer while codewords for a previous slice are being read out from the other buffer. A dual buffer arrangement of this type is used in the system described in the Raychaudhuri patent. Although the dual buffer technique works adequately, it is expensive in terms of hardware since two buffers are required. In addition, the circuitry for generating timing control signals is complicated since the slice codewords typically exhibit different lengths. Apparatus according to the present invention exhibits simpler, less expensive architecture compared to the apparatus used in the Raychaudhuri patent.